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Lucky Gunner’s 40000 Round 5.56 Ammunition Test

So here’s something awesome. The folks at Lucky Gunner decided to answer all the questions about brass versus steel cased 5.56×45 NATO ammunition in the AR-15 type with a forty thousand round torture test. They did a great job in setting the conditions, collecting the data, and presenting their analysis. I recommend you read the report yourself if you have an AR, but here’s some of the things I found interesting.

First, I’m impressed a Shrubmaster AR made it through ten thousand rounds without stoppages. I’d be even more impressed if the team hadn’t had to properly torque the barrel nuts on all the rifles! While I guess things are improving, there’s really no reason to buy a bottom quality AR like a Bushmaster, DPMS, Armalite, Olympic, RRA or others when rifles like the Colt 6920, S&W M&P-15, Daniel Defense, or BCM are available at about the same price, or a Knights or Noveske at a bit more. You only have to buy it once! ["But pdb, didn't you just assemble two 16" ARs with uppers from a questionable parts house and mismatched lowers from Brownells?" Okay, let me explain that. SHUT UP.]

I also did not expect the degree of throat and gas port erosion discovered when the team sectioned the barrels of the steel-cased shooting guns. While this was not observed in the rifle shooting brass cased ammunition, I do believe they aggravated the issue by subjecting the rifles to firing schedules normally associated with belt-fed machine guns. While the team did take steps to monitor the exterior barrel temperature, when you’re repeatedly heating steel up to where it becomes elastic, bad things happen. It does appear that shooting a lot of steel cased ammo is indeed harder on your barrel, but you can make it significantly worse if you shoot it very fast.

I expected more visible damage to the extractors, since that’s what we keep warning people about. But as it turns out, the differences between them were not detectable with the naked eye! Still, an extractor is a $15 part and takes minutes to change. You should know how to do this and keep a spare handy.

I think the biggest lesson here is that if you treat your AR like the high-volume gun it is, then you need to consider barrels, bolts and extractors as wear parts and plan for their replacement if you intend to take the gun into five figures of rounds fired. But realistically, most shooters aren’t going to shoot that much, and indeed have little need to since it is easier to maintain rifle proficiency, with fewer rounds, than it is to keep sharp on the pistol. And certainly you’re not learning a whole lot nor improving your skills much with repeated mag dumps, so this concern is largely irrelevant to most shooters.

I do shoot a fair amount of steel cased ammunition in my ARs, but I think I will now stick to brass ammunition in my Mixmaster A1, since the Colt 1:12 pencil barrel is going to be difficult to replace. I’d also recommend going exclusively brass if you have a nice stainless steel or match barrel. But I’m not giving up on steel in my other rifles, especially for practice fodder, because the cost savings are significant. I would not recommend using steel cased ammunition for self defense or competition, and think it’s a good idea to limit high-volume, high-intensity shooting with steel.

So I learned a lot, and I’m glad they spent the money and went through the effort to teach us a few things. I’m sure it’ll be useful information when we’re all able to buy ammunition again.

{ 7 } Comments

  1. aczarnowski | January 9, 2013 at 9:21 am | Permalink

    My take away from that great test was the jacket material is what matters, not the case. Like a lot of things, after you see it it’s obvious that bi-metal jackets would be harder on barrels than straight copper. So because comm-block steel cased ammo generally gets you bi-metal jackets for free, yes, “watch the steel case intake” is a good rule of thumb.

    But I’ll be watching the bullet jacket material more closely than the case material myself.

  2. pdb | January 9, 2013 at 5:02 pm | Permalink

    Partially. But steel being far less elastic than brass also contributed to significantly higher throat erosion at the chamber end.

  3. Andrew Tuohy | January 9, 2013 at 5:10 pm | Permalink

    To be clear, the Federal carbine had greater gas port erosion than the Wolf or Brown Bear carbines.

    The rate of fire was the same as the 2003 CQBR test:

    However, we managed lower temperatures due to semi auto fire.

  4. Rick R | January 9, 2013 at 6:11 pm | Permalink

    I’m not too familiar with AR’s but I do have some experience with throat erosion from my use of long range rifles. HEAT is the big culprit. The rate of fire is the largest factor with the second being the type of powder you are using. Hyper-Velocity varmint cartridges can burn out a throat in 1000 rounds, 500 if you shoot quickly.

  5. Anon | January 9, 2013 at 7:31 pm | Permalink

    I kinda figured the elasticity difference between steel and brass was a big contributing factor in throat erosion. I keep a couple cases of Wolfe around for students who run short, because it’s cheap, and if they wanted to shoot nothing but brass, well, they were told what the round count was for class. Anyway, I’ve noticed more staining on steel cases in all calibers, especially the straight case stuff (9, 40, 45) and dirtier chambers .

    Thinking about writing LG and asking if what the numbers were between the beginning and ending chamber casts. I’d guess they’d see some dimension changes at the front of the chambers above the shoulders as well.

  6. bob r | January 10, 2013 at 11:57 am | Permalink

    Technical quibble: “elastic” does not mean what you appear to think it means.

    “… when you’re repeatedly heating steel up to where it becomes elastic, …”

    Heating steel will make it *less* elastic. I.e., it becomes more plastic. Temperature *is* a significant factor in the strength and how toughness of steel. Too hot it becomes plastic and also less tough. Too cold and it becomes brittle.

    “… But steel being far less elastic than brass …”

    Steel is *far* more elastic than brass. Because of this, brass will better conform to the shape of the chamber and seal better than a similar case made from steel.

    From Wikipedia article on Young’s modulus: steel has an elastic modulus of 210 GPa. Brass is 100 to 125 GPa.

  7. pdb | January 10, 2013 at 1:49 pm | Permalink

    Technical quibble: “elastic” does not mean what you appear to think it means.

    Oh sure, if we’re gonna go get all “accurate” and “science-y” and stuff.

    Thanks for the correction!

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  1. SayUncle » Wear and tear | January 9, 2013 at 11:51 am | Permalink

    [...] PDB has thoughts on LuckyGunner’s 40K ammo comparison. [...]